The present invention relates to compositions comprising free radical polyethylene and ethylene/.alpha.-olefin copolymers and also to the application of the compositions to the manufacture of films.
Compositions of free radical polyethylene (obtained by a high pressure process) and ethylene/.alpha.-olefin copolymers (obtained by a low pressure process) are already known. For example, European Patent Application No. 0,006,110 describes films, with a thickness between 5 and 500 microns, extruded from compositions comprising from 1 to 20 weight percent free radical polyethylene with a melt index between 0.2 and 5 dg/minute and from 80 to 99 weight percent ethylene/.alpha.-olefin copolymer with a specific gravity between 0.912 and 0.940, a melt index between 0.1 and 5 dg/minute, a polydispersity index (defined by the ratio Mw/Mn of the weight average molecular weight to the number-average molecular weight) between 2.7 and 4.1, and an overall degree of unsaturation of between 0.1 and 0.3 double bonds per 1,000 carbon atoms. The copolymer that forms part of these compositions is manufactured by a low pressure (10 to 70 bars) and low temperature (30.degree. C. to 115.degree. C.) process. Copolymers of this type have a perfectly homogeneous structure, as described in French Pat. No. 1,604,980. French Pat. No. 2,481,709 describes a mixture of: (a) 30 to 80 weight percent of an ethylene/.alpha.-olefin copolymer, the .alpha.-olefin having from 3 to 12 carbon atoms, and the copolymer having a specific gravity of less than 0.945 and an intrinsic viscosity at 135.degree. C. of 1.7 to 10 dl/g; and (b) 20 to 70 weight percent high pressure polyethylene having a specific gravity between 0.915 and 0.935 and an intrinsic viscosity at 135.degree. C. less than 1.3 dl/g. For an ethylene/.alpha.-olefin copolymer of this type, an intrisic viscosity at 135.degree. C. of more than 1.7 dl/g corresponds to a melt index (according to ASTM Standard Specification D 1238-73) of less than 0.3 dg/minute.
U.S. Pat. No. 3,176,052 describes a homogeneous mixture comprising: (a) from 5 to 75 weight percent free radical polyethylene having a specific gravity of 0.91 to 0.94; and (b) from 25 to 95 weight percent ethylene/.alpha.-olefin copolymer having a specific gravity greater than 0.92 and a melt index between 0.1 and 10 dg/minute. The patent teaches that the improvement in the gloss of an extruded film obtained from a mixture of this type, compared with the extruded films obtained from each constituent of the mixture, is more pronounced with a higher melt index of free radical polyethylene (3 to 10 dg/minute according to the examples) and with a lower melt index of the ethylene/.alpha.-olefin copolymer compared with the free radical polyethylene (from 0.5 to 1.4 dg/minute according to the examples).
Films extruded from free radical polyethylene having a thickness generally equal to at least 20 microns are also known.